Different methods for determining a state-of-charge of a battery are known from the prior art. In the case of one of these known methods, the battery state is determined by virtue of the fact that the voltage of the battery that is not under any load, also called OCV (open circuit voltage) is measured. In parts, this is compared in addition with the voltage of the battery when the battery is under a load. The method is relatively reliable when the state-of-charge has a steep dependency on the voltage of the battery that is not under any load, but it is unsuitable for use in battery systems that comprise a flat progression such as is illustrated in FIG. 1 using an example of a battery that is lithium-iron-phosphate based.
In the case of a further method for determining the state-of-charge of a battery, a battery current that is flowing through the battery is measured and integrated over time. A method of this type is used, for example, in battery systems that can be disconnected from the consumer network when not in use. An inactive operating mode of this type can then prevail if a superimposed control unit, for example an engine control unit in a motor vehicle, is switched off. If it is necessary for the battery system to meet a particular demand for a stand-by current, the battery can be noticeably discharged when the battery electronics are in an inactive operating mode of this type. In order for the state-of-charge to be determined in a sufficiently precise manner, it is necessary for a component that is performing the integration of the battery current over time to be permanently active which would lead to an increased discharge of the battery during the inactive operating mode.